There are many areas in which materials that provide thermal insulation are required. The exploration of space requires new technologies for long term cryogenic propellant storage applications in space, on the lunar surface, and on the earth. Thermal insulating materials help to lower the energy requirements to keep a substance hot or cold. High performance thermal insulation materials are needed to insulate cryotanks at both low and high temperatures on launch vehicles as well as cryogenic fluid storage tanks. Silica aerogels are the best known thermal insulating materials available. However, the mechanical strength of these aerogels needs to be improved to meet the requirements of these applications. Improvements in the strength of aerogels would allow these materials to be used as advanced non-compacting insulation materials capable of retaining structural integrity while accommodating larger operating temperatures ranging from cryogenic to elevated temperatures.
Silica containing aerogels crosslinked with trialkoxysilyl terminated organic compounds have been disclosed; see, for example, US Patent Application No. 2006/0286360 to Rhine et al. The mechanical strength of aerogel materials can be increased by reinforcing them with organic crosslinking agents. For example, polyimide materials have excellent thermal, mechanical and electronic properties compared to other organic polymeric materials, due to the highly rigid molecular structures. U.S. Pat. No. 7,074,880 to Rhine et al. discloses polymeric imides to which poly(dimethylsiloxane) has been attached and U.S. Pat. No. 7,074,880 to Rhine et al. discloses polyimides which are modified with silica, alumina and the like. In these disclosures, a majority of the materials are organic in nature. As such they deviate significantly from silica based aerogels. Also, it is not clear what properties a polyimide-silica composite would have in terms of its mechanical strength in combination with its thermal insulating properties.
Bis-trialkoxysilane diimide materials have been as an additive in polyimide coatings to improve their thermal and mechanical properties. Incompatibility issues control the types and amounts of silane material that can be used. These materials and coatings made therefrom are readily distinguishable from aerogels. Aerogels describe a class of material based upon their structure, namely low density, open cell structures, large surface areas (often 900 m2/g or higher) and sub-nanometer scale pore sizes. Supercritical and subcritical fluid extraction technologies are commonly used to extract the fluid from the fragile cells of the material. A variety of different aerogel compositions are known and they may be inorganic, organic and inorganic/organic hybrid (see N. Husing and U Schubert, Angew. Chem. Int. Ed. 1998, 37, 22-45). Inorganic aerogels are generally based upon metal alkoxides and include materials such as silica, carbides, and alumina. Organic aerogels include, but are not limited to, urethane aerogels, resorcinol formaldehyde aerogels, and polyimide aerogels. Organic/inorganic hybrid aerogel were mainly organically modified silicate. The organic components are covalently bonded to the silica network. In other words, the organic and inorganic phases are chemically bonded to each other in the inorganic/organic hybrid aerogels.
The strength of aerogels such as those based on polyimide can be increased by crosslinking the polyimides with triamines and alkoxysilanes, however the thermal conductivity properties suffer.
Therefore, there remains a need for light-weight silica aerogels which incorporate the excellent thermal conductivity properties of silica aerogels while incorporating the mechanical strength provided by organic crosslinking materials.